1. Field of the Invention
The present invention relates to an alkaline storage battery using a nickel electrode and provides an alkaline storage battery having a long life which is improved in charging receptivity after having been left to stand and thus being deeply discharged, thereby to heighten capacity recovery after charging and which can give a utilization ratio of positive electrode active material nearly the same as initial utilization ratio at the subsequent discharging.
2. Description of the Related Art
Non-sintered Ni electrodes used in alkaline storage batteries represented by nickel-hydrogen storage batteries and nickel-cadmium storage batteries have advantages of higher filling density of active material and simpler production process as compared with conventional sintered Ni electrodes, and are widely used at present.
Representative non-sintered Ni electrodes comprise a plaque of foamed or fibrous nonwoven fabric having a porosity of 90% or more, in which active material powders mainly composed of nickel hydroxide are filled. However, in the case of such non-sintered Ni electrodes, when only spherical nickel hydroxide powders having good filling properties are filled in spaces of plaque, no sufficient utilization ratio of active materials can be obtained since electrical conductivity of the plaque per se and electrodes is low. Therefore, it is necessary to heighten conductivity of the active materials to improve utilization ratio of active materials, and JP-A-62-237667 proposes to add cobalt compounds such as cobalt hydroxide powders and cobalt oxide powders as conductive agents to active materials.
When alkaline storage batteries are fabricated using these nickel electrodes comprising a plaque filled with a nickel hydroxide and a conductive agent such as cobalt hydroxide or cobalt oxide in the spaces thereof, the cobalt compound dissolves in an alkaline electrolyte as cobaltic acid ion, which uniformly disperses as cobalt hydroxide on the surface of nickel hydroxide. Thereafter, this cobalt hydroxide is oxidized to cobalt oxyhydroxide of high conductivity at the time of initial charging of batteries to form an electrically conductive network which connects between the active materials and between the active material and the porous plaque, and heightens the conductivity therebetween to improve utilization ratio of the active materials.
In recent electronic portable apparatuses, with increase of chances to use batteries as electric source, for example, in notebook-type personal computers, they are often left to stand for a long time in the state of the battery being connected to the circuit due to the inadvertent failure to shut off the power. If the apparatuses are left to stand for a long time in such state, the battery is discharged until the voltage reaches less than service voltage range (higher than 0.8 V), and after the capacity of the battery is lost, the battery is still in the state of being left to stand for a long time under discharging, namely, so-called deeply discharged state.
Since in the battery in deeply discharged state, potential of the positive electrode is lower than the reduction potential of cobalt oxyhydroxide (about 0 V for Hg/HgO electrode potential), there occurs a phenomenon that cobalt oxyhydroxide which forms the conductive network is reduced and dissolves out. Therefore, in the battery which is once in deeply discharged state, the conductive network formed between active materials and between the active material and the porous plaque is partially broken, and, hence, charging receptivity lowers and when the battery is again charged and discharged thereafter, utilization ratio of active material equal to initial one cannot be obtained.